In my prior co-pending application Ser. No. 148,129, filed May 28, 1871, I described and claimed some new 2-halo-.alpha.-(1'-sulfenylhalo-1'-methylethyl)-4-oxo-1-azetidineacetate ester and acid compounds of the formula ##SPC1##
Wherein R.sub.1 is an acyl group and R.sub.2 is hydrogen, or R.sub.1 and R.sub.2 taken together with the nitrogen to which they are bonded define a phthalimido or succinimido group; R.sub.3 is hydrogen or C.sub.1 to C.sub.4 -alkyl, trichloroethyl, benzyl, p-nitrobenzyl, p-methoxybenzyl, benzhydryl, phthalimido or phenacyl, X is chloro, bromo or C.sub.2 to C.sub.6 -alkanoyloxy, and Y is chloro, bromo, N-benzotriazolyl, or succinimido. Those compounds are prepared by selectively opening the S.sub.1 -C.sub.5 -bond of the thiazolidine ring of a 6-acylamido or a 6-imidopenicillin ester by reacting the penicillin ester with an electrophilic reagent in the form of a source of positive halogen in an aprotic solvent.